1. Field of the Invention
The present invention relates to a simple and sure evaluation process, by which a reactivity of a silica glass with silicon melt or a phenomenon of a vibration at the surface of the silicon melt held in a silica glass crucible can be evaluated. Furthermore, the present invention also relates to a silica glass crucible improved by applying said evaluation process. When the improved crucible is used for pulling silicon single crystal, a vibration at the surface of the silicon melt held in the crucible is remarkably suppressed.
2. Discussion of the Background
A silicon single crystal used as a raw material for a semiconductor silicon wafer has been produced mainly by Czochralsky (CZ) method. Since CZ method is the method of growing up the silicon single crystal, by contacting a single-crystal seed with the silicon melt held in the silica glass crucible, the crystallization of the silicon single crystal by CZ method is remarkably prevented by a phenomenon of the vibration at the surface of the silicon melt held in the silica glass crucible, and a yield of single crystal is decreased clearly. Hereinafter, the phenomenon is referred to as “the surface vibration of the silicon melt”. Therefore, the silica crucible, which does not cause the surface vibration of the silicon melt, has been required strongly.
The surface vibration of the silicon melt is caused by the reaction of the silica glass with the silicon melt. However, conventionally, as for the silica glass crucible, any method for evaluating the vibration level at the surface of silicon melt held in a silica glass crucible has not yet been known, and there has been no method to evaluate whether a silica glass crucible causes the vibration or not, except that a pulling test of silicon single crystal is actually carried out by using said silica glass crucible. Moreover, as for the reactivity of the silica glass with the silicon melt, which causes the surface vibration of the silicon melt, many complicated factors are related, for example such as a dissolution speed of silica glass to silicon melt, a bubble content of the crucible, a various impurities concentrations in silica glass, and a roughness on the surface of silica glass, and it has not been confirmed correctly that how degrees of the influence are given to the reactivity by each factors. Therefore, in order to obtain the silica crucible not causing the surface vibration of the silicon melt, it is necessary to carry out the many pulling tests to evaluate the crucibles, and even if said pulling tests are repeated, it is difficult to obtain the silica crucible not causing the surface vibration of the silicon melt.
In such a prior circumstances, recently, a method has been disclosed as the method for evaluating the surface vibration of the silicon melt, wherein the method consisted of melting silicon on a silica glass sample piece and measuring the decreasing amount of the silica glass to estimate indirectly the surface vibration of the silicon melt (Japanese Patent Laid Open No. 2002-154894). However, as for this method, an error is much, so that it is difficult to obtain the result agreeing with the result of pulling actually. As described above, many factors, such as the dissolution speed of the silica glass, the bubble content of the crucible, the various impurities concentrations, and the roughness of the surface, are related complicatedly to the surface vibration of the silicon melt, so that, only with measuring the decreasing amount of the silica glass as disclosed by the above prior art, it is difficult to evaluate the surface vibration of the silicon melt.